It is known that the rotating speed of a motor is controlled by modulating the amplitude of the voltage applied to the motor by using PWM (Pulse Width Modulation). For PWM control, the amplitude of the control signal that controls motor-driving is modulated to the pulse width, and applying the driving voltage signal that is modulated using PWM to the motor allows the power provided to the motor to be controlled, thereby controlling the rotating speed.
FIG. 4(a) is a schematic view of the motor control using PWM control. In the PWM control of the motor shown in FIG. 4(a), the amplitude of a driving command signal that controls the rotation in a PWM modulator 101 is modulated to the pulse width using PWM, and then the modulated signal by using pulse width modulation is applied to the motor 20.
It is known that the rotating speed varies as a result of ripple of the driving voltage in the motor control using PWM control.
When the driving voltage (PWM signal) obtained using PWM modulation is applied to the motor, a smoothed driving current flows in the motor. Then, the ripple of the driving current of the motor is generated as a result of the variation of the driving current while the driving current follows a change of the driving voltage that is a PWM signal. The ripple of the driving current increases with the conversion of the driving voltage. FIGS. 6(a) and (b) explain the ripple that is generated during PWM modulation. When the amplitude of the driving voltage is small, the ripple is small due to the small voltage variation. However, when the amplitude of the driving voltage is large, the ripple increases due to the large voltage variation. The broken lines in FIGS. 6(a) and (b) show one example of the ripple.
A technology that allows torque ripple to decrease in the motor control by using a Δ-Σ modulation has been known (Patent document 1).
Patent document 1 discloses a technique that reduces the torque ripple that appears in an offset due to a switching delay of comparators and/or MOSFETs constituting PWM circuits, by applying a negative feedback of the output signals of Δ-Σ modulation at the time of applying the voltage to the coil in the motor-driving by the PWM modulation.